Pegram prepared a letter of introduction for Fermi to carry along to his appointment. It stated a hesitant case dense with hypotheticals:
Experiments in the physics laboratory at Columbia University reveal that conditions may be found under which the chemical element uranium may be able to liberate its large excess of atomic energy, and that this might mean the possibility that uranium might be used as an explosive that would liberate a million times as much energy per pound as any known explosive. My own feeling is that the probabilities are against this, but my colleagues and I think that the bare possibility should not be disregarded.1122
Thus lightly armed, Fermi departed to engage the Navy.
The debate was hardly ended, nor Wigner’s long day done. He returned to Princeton with Szilard in tow for an important meeting with Niels Bohr. It had been planned in advance; John Wheeler and Léon Rosenfeld would attend and Teller was making a special trip up from Washington. If Bohr could be convinced to swing his prestige behind secrecy, the campaign to isolate German nuclear physics research might work.
They met in the evening in Wigner’s office. “Szilard outlined the Columbia data,” Wheeler reports, “and the preliminary indications from it that at least two secondary neutrons emerge from each neutron-induced fission. Did this not mean that a nuclear explosive was certainly possible?” Not necessarily, Bohr countered.1123 “We tried to convince him,” Teller writes, “that we should go ahead with fission research but we should not publish the results. We should keep the results secret, lest the Nazis learn of them and produce nuclear explosions first. Bohr insisted that we would never succeed in producing nuclear energy and he also insisted that secrecy must never be introduced into physics.”1124
Bohr’s skepticism, says Wheeler, concerned “the enormous difficulty of separating the necessary quantities of U235.”1125 Fermi noted in a later lecture that “it was not very clear [in 1939] that the job of separating large amounts of uranium 235 was one that could be taken seriously.”1126 At the Princeton meeting, Teller remembers, Bohr insisted that “it can never be done unless you turn the United States into one huge factory.”1127
More crucial for Bohr was the issue of secrecy. He had worked for decades to shape physics into an international community, a model within its limited franchise of what a peaceful, politically united world might be. Openness was its fragile, essential charter, an operational necessity, as freedom of speech is an operational necessity to a democracy. Complete openness enforced absolute honesty: the scientist reported all his results, favorable and unfavorable, where all could read them, making possible the ongoing correction of error. Secrecy would revoke that charter and subordinate science as a political system—Polanyi’s “republic”—to the anarchic competition of the nation-states. No one was more anguished than Bohr by the menace of Nazi Germany; Laura Fermi remembers of this period, “two months after his landing in the United States,” that “he spoke about the doom of Europe in increasingly apocalyptic terms, and his face was that of a man haunted by one idea.”1128 If U235 could be separated easily from U238, that misfortune might be cause for temporary compromise with principle in the interest of survival. Bohr thought the technology looked not even remotely accessible. The meeting dragged on inconclusively past midnight.
The next afternoon Fermi turned up at the Navy Department on Constitution Avenue for his appointment with Admiral Hooper. He had probably planned a conservative presentation. The contempt of the desk officer who went in to announce him to the admiral encouraged that approach. “There’s a wop outside,” Fermi overheard the man say.1129 So much for the authority of the Nobel Prize.
In what Lewis Strauss, by now a Navy volunteer, calls “a ramshackle old board room” Hooper assembled an audience of naval officers, officers from the Army’s Bureau of Ordnance and two civilian scientists attached to the Naval Research Laboratory.1130 One of the civilians, a bluff physicist named Ross Gunn, had watched Richard Roberts demonstrate fission in the target room of the 5 MV Van de Graaff at the DTM not long after Fermi passed through at the time of the Fifth Washington Conference. Gunn worked on submarine propulsion; he was eager to learn more about an energy source that burned no oxygen.
Fermi led his auditors through an hour of neutron physics. If the notes of one of the participants, a naval officer, are comprehensive, Fermi emphasized his water-tank measurements rather than Szilard’s more direct ionization-chamber work. New experiments in preparation might confirm a chain reaction, Fermi explained. The problem then would be to assemble a sufficiently large mass of uranium to capture and use the secondary neutrons before they escaped through the surface of the material.
The officer taking notes interrupted.1131 What might be the size of this mass? Would it fit into the breech of a gun?
Rather than look at physics down a gun barrel Fermi withdrew to the ultramundane. It might turn out to be the size of a small star, he said, smiling and knowing better.
Neutrons diffusing through a tank of water: it was all too vague. Except to alert Ross Gunn, the meeting came to nothing. “Enrico himself . . . doubted the relevance of his predictions,” says Laura Fermi.1132 The Navy reported itself interested in maintaining contact; representatives would undoubtedly visit the Columbia premises. Fermi smelled the condescension and cooled.
March 17 was a Friday; Szilard traveled down to Washington from Princeton with Teller; Fermi stayed the weekend. They got together, reports Szilard, “to discuss whether or not these things”—the Physical Review papers—“should be published. Both Teller and I thought that they should not. Fermi thought that they should. But after a long discussion, Fermi took the position that after all this was a democracy; if the majority was against publication, he would abide by the wish of the majority.”1133 Within a day or two the issue became moot. The group learned of the Joliot/von Halban/Kowarski paper, published in Nature on March 18.1134 “From that moment on,” Szilard notes, “Fermi was adamant that withholding publication made no sense.”1135
The following month, on April 22, Joliot, von Halban and Kowarski published a second paper in Nature concerning secondary neutrons.1136 This one, “Number of neutrons liberated in the nuclear fission of uranium,” rang bells. Calculating on the basis of the experiment previously reported, the French team found 3.5 secondary neutrons per fission. “The interest of the phenomenon discussed here as a means of producing a chain of nuclear reactions,” the three men wrote, “was already mentioned in our previous letter.” Now they concluded that if a sufficient amount of uranium were immersed in a suitable moderator, “the fission chain will perpetuate itself and break up only after reaching the walls limiting the medium. Our experimental results show that this condition will most probably be satisfied.”1137 That is, uranium would most probably chain-react.
Joliot’s was an authoritative voice. G. P. Thomson, J.J.’s son, who was professor of physics at Imperial College, London, heard it. “I began to consider carrying out certain experiments with uranium,” he told a correspondent later. “What I had in mind was something rather more than a piece of pure research, for at the back of my thoughts there lay the possibility of a weapon.” He applied forthwith to the British Air Ministry for a ton of uranium oxide, “ashamed of putting forward a proposal apparently so absurd.”1138
More ominously, two initiatives originated simultaneously in Germany as a result of the French report.1139 A physicist at Göttingen alerted the Reich Ministry of Education. That led to a secret conference in Berlin on April 29, which led in turn to a research program, a ban on uranium exports and provision for supplies of radium from the Czechoslovakian mines at Joachimsthal. (Otto Hahn was invited to the conference but arranged to be elsewhere.) The same week a young physicist working at Hamburg, Paul Harteck, wrote a letter jointly with his assistant to the German War Office:
We take the liberty of calling to your attention the newest development in nuclear physics, which, in our opinion, will probably make it possible to produce an explosive many orde
rs of magnitude more powerful than the conventional ones. . . . That country which first makes use of it has an unsurpassable advantage over the others.1140
The Harteck letter reached Kurt Diebner, a competent nuclear physicist stuck unhappily in the Wehrmacht’s ordnance department studying high explosives. Diebner carried it to Hans Geiger. Geiger recommended pursuing the research. The War Office agreed.
A public debate in Washington on April 29 paralleled the secret conference in Berlin. The New York Times account accurately summarizes the divisions in the U.S. physics community at the time:
Tempers and temperatures increased visibly today among members of the American Physical Society as they closed their Spring meeting with arguments over the probability of some scientist blowing up a sizable portion of the earth with a tiny bit of uranium, the element which produces radium.1141
Dr. Niels Bohr of Copenhagen, a colleague of Dr. Albert Einstein at the Institute for Advanced Study, Princeton, N.J., declared that bombardment of a small amount of the pure Isotope U235 of uranium with slow neutron particles of atoms would start a “chain reaction” or atomic explosion sufficiently great to blow up a laboratory and the surrounding country for many miles.
Many physicists declared, however, that it would be difficult, if not impossible, to separate Isotope 235 from the more abundant Isotope 238. The Isotope 235 is only 1 per cent of the uranium element.
Dr. L. Onsager of Yale University described, however, a new apparatus in which, according to his calculations, the isotopes of elements can be separated in gaseous form in tubes which are cooled on one side and heated to high temperatures on the other.
Other physicists argued that such a process would be almost prohibitively expensive and that the yield of Isotope 235 would be infinitesimally small. Nevertheless, they pointed out that, if Dr. Onsager’s process of separation should work, the creation of a nuclear explosion which would wreck as large an area as New York City would be comparatively easy. A single neutron particle, striking the nucleus of a uranium atom, they declared, would be sufficient to set off a chain reaction of millions of other atoms.
The Times story assumes the truth of Bohr’s argument in favor of U235, although even Bohr was apparently still emphasizing only a slowneutron reaction. Fermi and others were not yet convinced of U235’s role. The two uranium isotopes might not easily be separated in quantity, but it had occurred to John Dunning earlier in the month that they could be separated in microscopic amounts in Alfred Nier’s mass spectrograph. Dunning had immediately written Nier a long, impassioned letter asking him, in effect, to resolve the dispute between Fermi and Bohr and push chain-reaction research dramatically forward. Nier, Dunning and Fermi all attended the American Physical Society meeting. In person Dunning urged Nier to try for a separation much as he had urged him in the key paragraph of his letter:
There is one line of attack that deserves strong effort, and that is where we need your cooperation. . . . It is of the utmost importance to get some uranium isotopes separated in enough quantities for a real test. If you could separate effectively even tiny amounts of the two main isotopes [a third isotope, U234, is present in natural uranium to the trace extent of one part in 17,000], there is a good chance that [Eugene T.] Booth and I could demonstrate, by bombarding them with the cyclotron, which isotope is responsible. There is no other way to settle this business. If we could all cooperate and you aid us by separating some samples, then we could, by combining forces, settle the whole matter.1142
The important point for Dunning, the reason for his passion, was that if U235 was responsible for slow-neutron fission, then its fission cross section must be 139 times as large as the slow-neutron fission cross section of natural uranium, since it was present in the natural substance to the extent of only one part in 140. “By separating the 235 isotope,” Herbert Anderson emphasizes in a memoir, “it would be much easier to obtain the chain reaction. More than this, with the separated isotope the prospect for a bomb with unprecedented explosive power would be very great.”1143
Fermi urged Nier in similar terms; Nier recalls that he “went back and figured out how we might soup up our apparatus some in order to increase the output. . . . I did work on the problem, but at first it seemed like such a farfetched thing that I didn’t work on it as hard as I might have. It was just one of a number of things I was trying to do.”1144
Fermi in any case was more interested in pursuing a chain reaction in natural uranium than in attempting to separate isotopes. “He was not discouraged by the small cross-section for fission in the natural [element],” comments Anderson. “ ‘Stay with me,’ he advised, ‘we’ll work with natural uranium. You’ll see. We’ll be the first to make the chain reaction.’ I stuck with Fermi.”1145
By mid-April Szilard had managed to borrow about five hundred pounds of black, grimy uranium oxide free of charge from the Eldorado Radium Corporation, an organization owned by the Russian-born Pregel brothers, Boris and Alexander. Boris had studied at the Radium Institute in Paris; Eldorado speculated in rare minerals and owned important uranium deposits at Great Bear Lake in the Northwest Territories of Canada.
Like Fermi’s and Anderson’s previous experiment, the new project involved measuring neutron production in a tank of liquid. For a more accurate reading the experimenters needed a longer exposure time than their customary rhodium foils activated to 44-second half-life would allow. They planned instead simply to fill the tank with a 10 percent solution of manganese, an ironlike metal that gives amethyst its purple color and that activates upon neutron bombardment to an isotope with a nearly 3-hour half-life. “The [radio]activity induced in manganese,” they explained afterward in their report, “is proportional to the number of [slow] neutrons present.”1146 So the hydrogen in the water would serve to slow both the primary neutrons from the central neutron source and any secondary neutrons from fission, and the manganese in the water would serve to measure them—a nice economy of design.
Atoms on the surface of a mass of uranium are exposed to neutrons more efficiently than atoms deeper inside. Fermi and Szilard therefore decided not to bulk their five hundred pounds of uranium oxide into one large container but to distribute it throughout the tank by packing it into fifty-two cans as tall and narrow as lengths of pipe—two inches in diameter and two feet long.
Packing cans and mixing manganese solutions, which had to be changed and the manganese concentrated after each experimental run, was work. So was staying up half the night taking readings of manganese radioactivity. Fermi accepted the challenge with gusto. “He liked to work harder than anyone else,” Anderson notes, “but everyone worked very hard.” Except Szilard. “Szilard thought he ought to spend his time thinking.”1147 Fermi was insulted. “Szilard made a mortal sin,” Segrè remembers, echoing Fermi. “He said, ‘Oh, I don’t want to work and dirty my hands like a painter’s assistant.’ ”1148 When Szilard announced that he had hired a standin, a young man whom Anderson remembers as “very competent,” Fermi acceded to the arrangement without comment.1149 But he never again pursued an experiment jointly with Szilard.
The arrangement as finally consummated looked like this:
Szilard’s Ra + Be source stands in the center of the tank, which holds 143 gallons of manganese solution; the fifty-two cans of UO2 gather around.
It worked. The three physicists found neutron activity “about ten percent higher with uranium oxide than without it. This result shows that in our arrangement more neutrons are emitted by uranium than are absorbed by uranium.”1150 But the experiment raised puzzling questions. Resonance absorption, for example, was clearly a problem, capturing neutrons that might otherwise serve the chain reaction. The report estimates “an average emission [of secondary neutrons] of about 1.2 neutrons per thermal neutron” but notes that “this number should be increased, to perhaps 1.5,” because some of the neutrons had obviously been captured without fissioning—demonstrating the big capture resonance around 25 eV that Bohr had attributed on his
graphs to U238.1151
Another problem was the use of water as a moderator. As Fermi’s team had discovered in Rome in 1934, hydrogen was more efficient than any other element at slowing down neutrons, and slow neutrons avoided the parasitic capture resonance of U238. But hydrogen itself also absorbed some slow neutrons, reducing further the number available for fission. And it was already clear that every possible secondary neutron would have to be husbanded carefully if a chain reaction was to be initiated in natural uranium. George Placzek came down from Cornell, where he had found a new home, for a visit, looked over the arrangement and insightfully foreclosed its future. As Szilard tells it:
We were inclined to conclude that . . . the water-uranium system would sustain a chain reaction. . . . Placzek said that our conclusion was wrong because in order to make a chain reaction go, we would have to eliminate the absorption of [neutrons by the] water; that is, we would have to reduce the amount of water in the system, and if we reduced the water in the system, we would increase the parasitic absorption of [neutrons by] uranium [because with less water fewer neutrons would be slowed]. He recommended that we abandon the water-uranium system and use helium for slowing down the neutrons. To Fermi this sounded funny, and Fermi referred to helium thereafter invariably as Placzek’s helium.1152
In June the Columbia team wrote up its experiment and sent the resulting paper, “Neutron production and absorption in uranium,” to the Physical Review, which received it on July 3.1153 Fermi left for the Summer School of Theoretical Physics at Ann Arbor, his attention diverted, says Anderson, “by an interesting problem in cosmic rays.”1154 Either Fermi did not share Szilard’s sense of the urgency of chain-reaction research or he was withdrawing for a time from the Navy’s indifference and Placzek’s persuasive criticism of his uranium-water system; probably both. Anderson settled down to study resonance absorption in uranium, a project that would evolve into his doctoral dissertation.
Making of the Atomic Bomb Page 41
